1. Field of the Invention
The present invention relates to an exhaust gas purifying system for an internal combustion engine and particularly to an exhaust gas purifying system for an internal combustion engine, which has an improved capability of purifying hydrocarbons (referred to as HC hereinafter) in the exhaust gas at the start of the internal combustion engine.
2. Description of the Related Art
A variety of studies for purifying exhaust gases from an internal combustion engine e.g. of an automotive vehicle have been developed to prevent pollution of the atmosphere. Known harmful substances in the exhaust gas from the vehicle are HC, CO, and NOx. For example, one of schemes for purifying HC is disclosed in Japanese Patent Laid-open Publication Hei 5-59942.
Disclosed in the Publication is a conventional exhaust gas purifying system which includes, in this order from the upstream of an exhaust gas passage of the internal combustion engine, a first three-way catalyst, an HC adsorbent, and a second three-way catalyst for purifying HC while the temperature of the gas is relatively low just after the start of the engine. As well known, three-way catalyst has a lower capability of reactively purifying HC before its temperature rises up to an activating level (300.degree. C.). Its capability will then increase as the temperature reaches the activation level. HC adsorbent has a higher capability of adsorbing HC when the temperature is low. When its temperature exceeds a certain level, the HC adsorbent starts desorbing HC. In the conventional exhaust gas purifying system, the first three-way catalyst is provided for extending a period before the temperature of the HC adsorbent reaches the certain level. During the extended period, the three-way catalyst is heated up to improve the purification of HC at the start of the engine.
Another scheme is disclosed in Japanese Patent Laid-open Publication Hei 5-31359. The scheme has a zeolite adsorbent, a honeycomb heater, and a main monolithic catalyst provided in an exhaust gas passage of an automotive vehicle so that HC in the exhaust gas of a lower temperature at the start of an engine is adsorbed by the adsorbing action of the zeolite adsorbent and further purified through energizing the honeycomb heater to instantaneously activate the catalyst on the heater.
However, in the former conventional system, the speed of rising the temperature of the exhaust gas is slowed down by the first three-way catalyst located at the upstream side. The speed of rising the temperature of the exhaust gas received by the HC adsorbent is not successfully retarded and the period before the temperature rises up to the level at which HC is desorbed from the HC adsorbent can hardly be extended to desired length.
In the latter conventional system, the catalyst is activated by heating with the honeycomb heater before the desorption of HC from the zeolite adsorbent starts. Since the catalyst is heated up within a short time, the supply of electric power to the honeycomb heater has to be increased.